AGC detector and method for AGC detecting

ABSTRACT

Disclosed is an AGC detector device and an AGC detecting method for television receivers displaying video pictures consisting of a plurality of horizontal lines to be built up successively, wherein a CVBS signal is inputted which includes horizontal sync pulses having a front porch region and a back porch region and occurring once a horizontal line during a horizontal sync period when generating a current video picture, and further includes vertical sync pulses occurring during a vertical sync period before the generation of a new video picture and including serration pulses which occur during a serration pulse region being part of the vertical sync period. Further, gating pulses are generated having a period which is equal to the line period of the horizontal sync pulses. Said gating pulses are adjusted such that they occur at the back porch region of the horizontal sync pulses. The particularity of the invention is that the occurrence of the gating pulses is shifted during the serration pulse region of the vertical sync period so that the gating pulses occur earlier by a time interval corresponding to the time distance between a front porch region and the next back porch region or later by a time interval corresponding to the time distance between a back porch region and the next front porch region.

The present invention relates to an AGC (automatic gain control)detector device for television receivers displaying video picturesconsisting of a plurality of horizontal lines to be built upsuccessively, comprising

means for inputting a CVBS (color video blanking sync) signal whichincludes horizontal sync pulses having a front porch region and a backporch region and occurring once a horizontal line during a horizontalsync period when generating a current video picture, and furtherincludes vertical sync pulses occurring during a vertical sync periodbefore the generation of a new video picture and including serrationpulses which occur during a serration pulse region being part of thevertical sync period,

means for generating gating pulses having a period which is equal to theline period of the horizontal sync pulses, and

means for adjusting said gating pulses such that they occur at the backporch region of the horizontal sync pulses.

Further, the present invention relates to a method for AGC detecting fortelevision receivers displaying video pictures consisting of a pluralityof horizontal lines to be built up successively, comprising the steps of

inputting a CVBS signal which includes horizontal sync pulses having afront porch region and a back porch region and occurring once ahorizontal line during a horizontal sync period when generating acurrent video picture, and further includes vertical sync pulsesoccurring during a vertical sync period before the generation of a newvideo picture and including serration pulses which occur during aserration pulse region being part of the vertical sync period,

generating gating pulses having a period which is equal to the lineperiod of the horizontal sync pulses, and

adjusting said gating pulses such that they occur at the back porchregion of the horizontal sync pulses.

A so-called digital black level (back porch) automatic gain control fortelevision receivers is already known, wherein the horizontal linefrequency is used to determine the level at the back porch region of avideo output signal. This is compared to a reference level for black,and the difference is the resulting loop gain error. The resulting loopgain error is integrated, digital/analog-converted and finally appliedto an analog AGC amplifier via an analog non-critical first-order postfilter in order to keep the video output voltage constant.

However, in some particular conditions the video output voltage isaffected by a negative effect called “airplane flutter” which resultsfrom changing multi-path reception. In particular, due to this negativeeffect the video output voltage is not kept on a constant level anytime, but varying, and sometimes the video output voltage is completelydistorted. This negative effect leads to a considerable reduction ofpicture and sound quality.

The serration pulse region which is part of the vertical sync period(=19 to 25 line periods depending on the television standard) is aparticularly critical period, because the serration pulses have no backporch region, rather the sync peak, but the gating pulses aresynchronized in accordance with the back porch region of the horizontalsync pulses.

Therefore, in the conventional AGC detectors the horizontal gatingprocedure is kept or freezed during the above critical period. However,this produces visible interferences in the picture under theabove-mentioned field conditions.

Also known is the disabling of the gating during the serration pulseregion. However, the result is a wrong AGC action in an annoying way incase of a fast AGC response requirement. Namely, when the gating isinterrupted during the serration pulse region, which lasts 2.5 to 3 lineperiods for all television standards, the automatic gain control cannotreact to the airplane flutter effect during that time period. As aconsequence the video output level will vary accordingly resulting in achanging picture contrast or even loss of synchronization.

It is an object of the present application to provide an automatic gaincontrol detector device and a method for automatic gain controldetecting which are fast enough to follow airplane flutter receptionconditions with still good picture and sound impression.

In order to achieve the above and further objects, in accordance with afirst aspect of the present invention, there is provided an AGC detectordevice for television receivers displaying video pictures consisting ofa plurality of horizontal lines to be built up successively, comprising

means for inputting a CVBS signal which includes horizontal sync pulseshaving a front porch region and a back porch region and occurring once ahorizontal line during a horizontal sync period when generating acurrent video picture, and further includes vertical sync pulsesoccurring during a vertical sync period before the generation of a newvideo picture and including serration pulses which occur during aserration pulse region being part of the vertical sync period,

means for generating gating pulses having a period which is equal to theline period of the horizontal sync pulses, and

means for adjusting said gating pulses such that they occur at the backporch region of the horizontal sync pulses, characterized by means forshifting the occurrence of said gating pulses during the serration pulseregion of the vertical sync period so that the gating pulses occurearlier by a time interval corresponding to the time distance between afront porch region and the next back porch region or later by a timeinterval corresponding to the time distance between a back porch regionand the next front porch region.

In accordance with a second aspect of the present invention, there isprovided a method for AGC detecting for television receivers displayingvideo pictures consisting of a plurality of horizontal lines to be builtup successively, comprising the steps of

inputting a CVBS signal which includes horizontal sync pulses having afront porch region and a back porch region and occurring once ahorizontal line during a horizontal sync period when generating acurrent video picture, and further includes vertical sync pulsesoccurring during a vertical sync period before the generation of a newvideo picture and including serration pulses which occur during aserration pulse region being part of the vertical sync period,

generating gating pulses having a period which is equal to the lineperiod of the horizontal sync pulses, and

adjusting said gating pulses such that they occur at the back porchregion of the horizontal sync pulses, characterized by the further stepof

shifting the occurrence of said gating pulses during the serration pulseregion of the vertical sync period so that the gating pulses occurearlier by a time interval corresponding to the time distance between afront porch region and the next back porch region or later by a timeinterval corresponding to the time distance between a back porch regionand the next front porch region.

So, the present invention provides a new AGC detector device and a newmethod for AGC detecting which are fast enough to follow airplaneflutter reception conditions with still good picture and soundimpression. This is achieved by gating the serration pulse region withmultiple (in particular three) time shifted horizontal sync pulses andby taking the error signal from the front porch measurement. In otherwords, the gating is changed from back porch to front porch during theserration pulse region of the vertical sync period which lasts 19 to 25line periods depending on the television standard. As a result, obtainedis a high speed response which is necessary to control the video outputsignal during airplane flutter and, thus, to keep it on a constant levelanytime. So, it is assured by the present invention that the AGC loopstays closed all the time and can, thus, function with good performance.

After all, an advantage of the present invention is that televisionreceivers create good picture and sound impression even under highvelocity e.g. in cars. So, the present invention is in particularconvenient for mobile television.

Further advantageous embodiments of the present invention are defined inthe dependent claims.

Preferably, provided are first means for detecting the occurrence of theback porch region of the horizontal sync pulses, and second means fordetecting the occurrence of the front porch region of the horizontalsync pulses, wherein said first and second detecting means are coupledto said shifting means and at least said first detecting means iscoupled to said adjusting means.

In a still further preferred embodiment, the device is a digital AGCdetector device. Namely, the change of the gating from back porch tofront porch during the serration pulse region can be done easily becauseit can happen in the digital domain.

In particular, the present invention can be provided for analogtelevision receivers.

Preferably, the gating pulses generating means comprises a PLL circuitwhich can be a horizontal PLL circuit, and the gating pulses generatingmeans can further comprise a vertical integrator. Namely, it isadvantageous to supply the gating pulses from a H-PLL and a V-INTEGRATORfor terrestrial noisy conditions.

For good terrestrial picture conditions and in a VCR (video cassetterecorder)/STB (set top box) mode via an RF modulator, the gating pulsescan be also derived from a conventional sync slicer.

When copy protection (e.g. MACROVISION) in the VCR/STB mode is set, somegating pulses are blanked.

In the following, the present invention will be described in greaterdetail based on a preferred embodiment with reference to theaccompanying drawings in which

FIG. 1 shows a schematic block diagram of a preferred embodiment of thea gated digital AGC circuit;

FIG. 2 is a graph showing the waveform of a CVBS signal without airplaneflutter;

FIG. 3 is a graph showing the waveform of a CVBS signal with no gatingpulses during the serration pulse region under the influence of airplaneflutter (prior art);

FIG. 4 is a graph showing the waveform of a CVBS signal with frontporchgating pulses during the serration pulse region under theoccurrence of airplane flutter;

FIG. 5 is a graph showing the waveform of the CVBS signal of FIG. 4 withadditional details of timing instants for sync and gating pulsesaccording to a first alternative of a method for changing the gatingfrom back porch to front porch; and

FIG. 6 is a graph showing the wave form of the CVBS signal of FIG. 4,but with additional details of timing instants for sync and gatingpulses according to a second alternative of a method for changing thegating from back porch to front porch.

A schematic block diagram of a preferred embodiment of a gated digitalAGC (automatic gain control) circuit for analog television receivers isshown in FIG. 1. The shown circuit comprises an input terminal 2 forinputting an IF (intermediate frequency) signal. The input terminal 2 isconnected to an input of a controllable analog AGC amplifier 4 whoseoutput is coupled to an output terminal 6 for outputting the gained IFsignal. The output of the analog AGC amplifier 4 is further connected toan input of an analog/digital-converter 8 whose output is coupled to aninput of a digital signal processing device 10. An output of the digitalsignal processing device 10 is connected to an input of a bit streamdigital/analog-converter 12 whose output is coupled via a feed back loop14 to an input of an analog non-critical first-order post filter 16. Theoutput signal of the post filter 16 is a control signal for controllingthe AGC amplifier 4.

In the digital signal processing device 10, a CVBS (color video blankingsync) signal is generated by demodulation of the inputted IF signalwhich has been digitized by the analog-digital-converter 8. Further, inthe digital signal processing device 10, the horizontal line frequencyis derived from the CVBS signal and is used to determine the level atthe back porch region of the CVBS signal. This level is compared to areference level for black. For this purpose, a H-PLL circuit is providedin the digital signal processing device 10. The difference between thelevel at the back porch region of the CVBS signal and the referencelevel for black is the resulting loop gain error which is integrated inthe digital signal processing device 10. So, gating pulses are producedby being supplied from a H-PLL and V-INTEGRATOR. However, this is mainlyfor terrestrial noisy conditions. For good terrestrial pictureconditions and in VCR (video cassette recorder)/STB (set top box) modevia an RF modulator, some gating pulses are derived from a conventionalsync slicer which is also included in the digital signal processingdevice 10. When a copy protection (e.g. MACROVISION) from VCR or STB isset, some gating pulses are blanked. For this purpose, a correspondingflag is set in the digital signal processing device 10.

A multi-bit AGC error signal is outputted from the digital signalprocessing device 10, converted into an analog signal by thedigital/analog-converter 12 and applied as an AGC error signal to theAGC amplifier 4 via the feed back loop 14 and the post filter 16 inorder to keep the output signal of the AGC amplifier 4 constant.

The occurrence of the gating pulses is synchronized with the back porchregion of the horizontal sync pulses. However, a critical time period isthe serration pulse region forming part of the vertical sync periodwherein serration pulses occur which have no back porch region. In FIG.2, it is shown the waveform of the CVBS signal without influence ofairplane flutter and in particular the waveform of the CVBS signalduring the beginning of the vertical sync period including the serrationpulse region.

State of the art is either keeping the horizontal gating procedure orfreezing it during the serration pulse region. Also known is thedisabling of the gating during the serration pulse region. The result isa wrong AGC action in an annoying way in case a fast AGC response isrequired. Namely, a high speed response is necessary to control theoutputted CVBS signal during airplane flutter (changing multi-pathreception) and keep it on a constant level anytime. However, when thegating is interrupted during the serration pulse region, which lasts 2.5to 3 line periods for all television standards, the AGC cannot react toairplane flutter during that critical time. As a consequence, the levelof the outputted CVBS signal will vary accordingly, resulting in achanging picture contrast or even loss of synchronization. Thissituation is depicted in FIG. 3 showing the waveform of the CVBS signalunder the influence of airplane flutter (80% AM, 80 Hz) and furthershowing the gating pulses indicated as upright arrows below the CVBSwaveform.

To overcome this unwanted effect, the digital signal processing device10 changes the gating from back porch to front porch during theserration pulse region. So, the serration pulse region is gated withthree time-shifted horizontal sync pulses by taking the error signalfrom the front porch measurement. In the digital domain this can be donewith high accuracy. So, the AGC loop stays closed all the time and canthus function with good performance. This situation is depicted in FIG.4 showing the waveform of the CVBS signal under the occurrence ofairplane flutter (80% AM, 80 Hz) and further showing the gaiting pulsesindicated as upright arrows below the CVBS waveform, wherein front porchgating is carried out during the serration pulse region.

FIG. 5 shows more details of the timing instants for the sync andgaiting pulses with reference to FIG. 4. In particular, FIG. 5 shows inmore detail that during back porch gating the gating pulses occur a timedistance ΔT1 after the occurrence of the sync pulses, whereas during theserration pulse region when front porch gating is carried out the gatingpulses occur a time distance ΔT2 before the occurrence of the syncpulses. So, according to FIG. 5 the gating pulses occur earlier by atime interval corresponding to the total time distance ΔT1+ΔT2 to hitthe front porch regions during the serration pulse region.

FIG. 6 shows an alternative method of changing from back porch gating tofront porch gating wherein the gating pulses are delayed by a timeinterval ΔT3 behind the occurrence of the sync pulses to hit the nextfront porch region and the subsequent front porch regions during theserration pulse region. So, in the situation of FIG. 6, the gatingpulses occur later by a time interval corresponding to the time distanceΔT3−ΔT1 between the first back porch region and the next front porchregion within the serration pulse region so as to change from back porchgating to front porch gating. With respect thereto, it should be addedthat apart from the different kind of changing from back porch gating tofront porch gating, the situation of FIG. 6 is the same as that of FIGS.4 and 5.

After all, FIGS. 4 to 6 clearly show that the airplane flutter does nothave any negative influence on the waveform of the CVBS signal.

As it becomes clear from the above description, the digital signalprocessing device 10 (FIG. 1) includes inter alia the function of an AGCdetector.

Finally, three general cases be distinguished:

1) Noisy Terrestrial Reception Mode:

The gating pulses are derived from a H-PLL and V-INTEGRATOR which areincluded in the digital signal processing device 10 (FIG. 1). Moreover,the AGC loop bandwidth is made small (reduced AGC speed) to cope withnoise spikes. The airplane flutter is masked by noise in that case.

2) Good Terrestrial Reception and VCR/STB Mode:

A threshold detector senses the AGC voltage in the digital domain tomonitor the IF (intermediate frequency) level. A flag is set, when theIF level is so high as to expect a good, nearly noise-free picturequality. During most of the active lines, carrying video information,the H-PLL supplies the gating pulses. However due to the VCR headswitching, even the fast H-PLL looses tracking for some lines because oftiming jitter. Therefore, an ordinary sync slicer must be employed inthe digital signal processing device 10 (FIG. 1). The VCR head switchinghappens before the vertical sync period. So, the sync slicer providesgating pulses for 20 line periods (as an example) plus the vertical syncperiod which results in 39 to 45 line periods in total.

3) Mode Like in 2), but with MACROVISION Copy Protection:

This situation is detected in a special circuit that finds out whetherany copy protection pulses occur. In that case, the H-PLL gating pulsesare interrupted during the vertical sync period (this is the periodduring which MACROVISION copy protection signals can occur). This can bedone, because no airplane flutter is expected in case of a MACROVISIONprotected signal.

Although, the invention is described above with reference to an exampleshown in the attached drawings, it is apparent that the invention is notrestricted to it, but can vary in many ways within the scope disclosedin the attached claims.

1. An AGC (automatic gain control) detector device for televisionreceivers displaying video pictures consisting of a plurality ofhorizontal lines to be built up successively, comprising means forinputting a CVBS (color video blanking sync) signal which includeshorizontal sync pulses having a front porch region and a back porchregion and occurring once a horizontal line during a horizontal syncperiod when generating a current video picture, and further includesvertical sync pulses occurring during a vertical sync period before thegeneration of a new video picture and including serration pulses whichoccur during a serration pulse region being part of the vertical syncperiod, means for generating gating pulses having a period which isequal to the line period of the horizontal sync pulses, and means foradjusting said gating pulses such that they occur at the back porchregion (ΔT1) relative to the horizontal sync pulses, characterized bymeans for shifting the occurrence of said gating pulses during theserration pulse region of the vertical sync period so that the gatingpulses occur earlier by a time interval corresponding to the timedistance (ΔT1+ΔT2) between a front porch region and the next back porchregion or later by a time interval corresponding to the time distance(ΔT3−ΔT1) between a back porch region and the next front porch region.2. The device according to claim 1, further characterized by first meansfor detecting the occurrence of the back porch region of the horizontalsync pulses, and second means for detecting the occurrence of the frontporch region of the horizontal sync pulses, wherein said first andsecond detecting means are coupled to said shifting means, and at leastsaid first detecting means is coupled to said adjusting means.
 3. Thedevice according to claim 1, wherein the device is a digital AGCdetector device.
 4. The device according to claim 1, wherein the deviceis provided for analog television receivers.
 5. The device according toclaim 1, wherein said gating pulses generating means comprises a PLLcircuit.
 6. The device according to claim 5, wherein said PLL circuit isH-PLL circuit, and said gating pulses generating means further comprisesa V-INTEGRATOR.
 7. The device according to at least any one of claim 1,wherein said gating pulses generating means comprises a sync slicer. 8.A method for AGC (automatic gain control) detecting for televisionreceivers displaying video pictures consisting of a plurality ofhorizontal lines to be built up successively, comprising the steps ofinputting a CVBS (color video blank sync) signal which includeshorizontal sync pulses having a front porch region and a back porchregion and occurring once a horizontal line during a horizontal syncperiod when generating a current video picture, and further includesvertical sync pulses occurring during a vertical sync period before thegeneration of a new video picture and including serration pulses whichoccur during a serration pulse region being part of the vertical syncperiod, generating gating pulses having a period which is equal to theline period of the horizontal sync pulses, and adjusting said gatingpulses such that they occur at the back porch region (ΔT1) relative tothe horizontal sync pulses, characterized by the further step ofshifting the occurrence of said gating pulses during the serration pulseregion of the vertical sync period so that the gating pulses occurearlier by a time interval corresponding to the time distance (ΔT1×ΔT2)between a front porch region and the next back porch region or later bya time interval corresponding to the time distance (ΔT3−ΔT1) between aback porch region and the next front porch region.
 9. The methodaccording to claim 8, wherein at least the step of shifting theoccurrence of said gating pulses during the serration pulse region iscarried out in the digital domain.
 10. The method according to claim 8,wherein the method is provided for analog television reception.